Package for perishable food and horticultural products

ABSTRACT

An improved package is described for use in packaging a wide variety of products, and in particular perishable food and horticultural products. The products are placed within a container of the type which establishes a modified atmosphere environment therein and is cooled, as by placing the container in contact with a cooling element partially surrounding the container. The cooling element may hold liquid which is evaporated to enhance the cooling. The container and cooling collar may be subjected to vacuum cooling. The container may include a mechanism to enhance the bulk gas transfer rate during the application of the vacuum while still maintaining the desired atmosphere within the container. The container atmosphere may be precharged with gas of a desired composition. In addition, fumigants may also be included therein. The package is suitable for field packing applications in which the product, such as strawberries, is picked directly into the container surrounded by the cooling collar with the container and cooling collar being placed in an outer box or other receptacle. The modified atmosphere package may be sealed and palletized prior to vacuum cooling.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/577,993 filed Sep. 5, 1990, to Floyd, et al. and entitled "APackage for Perishable Food and Horticultural Products" now U.S. Pat.No. 5,747,082 .

BACKGROUND OF THE INVENTION

The present invention relates to packaging for perishable products andin particular, to packaging usable in both cooling and protecting theproducts.

Several methods are commonly used for cooling perishable products whererapid cooling is required. These include hydrocooling, vacuum cooling,icing and forced air refrigeration. For example, the so-called "DesertWater Bag" operates on the principle that the evaporation of water fromfabric forming the bag cools the water in the bag.

In the produce field, it is common to pick heads of lettuce and placethem in waxed boxes with the box of lettuce then being hosed down withwater either before or after the boxes are loaded onto a truck. Althoughevaporation of water from the lettuce during transportation assists incooling the lettuce, relatively insignificant amounts of water areabsorbed by the waxed boxes and cooling is limited. Transportation ofbroccoli in waxed boxes filled with ice is also known.

In addition, vacuum cooling approaches have been used for coolingproduce. In accordance with this cooling technique, the warm product isloaded into an air tight chamber or tube which is subsequently evacuatedmy a mechanical or steam-ejector vacuum pump to establish a partialvacuum therein. As the total gas pressure in the tube is reduced belowthe saturation pressure of water at the temperature of the warm product(the "flash point"), water on and within the product begins to evaporaterapidly. The thermal energy required to provide the heat of vaporizationof this water comes predominately from the sensible heat (e.g. "fieldheat") of the product. As a result, the product temperature begins tofall as rapid evaporation begins. Because vacuum pumps are generallyvery inefficient movers of condensable gases, such as water vapor,chilled coils are provided within the tube or chamber to condense andthereby remove the liberated water vapor. These coils are chilledusually by evaporation of liquid ammonia within, the ammonia beingsupplied by a conventional vapor-compression refrigeration unit.

In the absence of air or any other restriction to water vapor movementfrom the product to the chilled coil, the temperature of the productwill in time equilibrate with that of the coil (the coil temperature infact being commonly used as a control variable in vacuum coolingoperations). Under these circumstances, the rate of thermalequilibration is largely determined by product characteristics. Ingeneral, products high in readily evaporated moisture content, with highthermal conductivity and high evaporative surface-to-volume ratio, willcool more rapidly under vacuum than do other types of products. Forexample, lettuce and other leafy vegetables cool well under vacuum (highmoisture content and high surface-to-volume ratio), while melons do not(low evaporation rate and low surface-to-volume ratio). In addition,strawberries have not been viewed as suitable for vacuum cooling becauseof damage to the surface of the berries under vacuum conditions and therelatively small rise in cooling rate resulting from the vacuumconditions as opposed to nonvacuum refrigeration type cooling.

One example of a prior art vacuum cooling system is described in U.S.Pat. No. 4,576,014 to Miller, et al. In these approaches, water has beenknown to be added to the produce by sprinkling the produce before orwhile the vacuum is imposed to reduce the amount of moisture removedfrom the produce during cooling with the water evaporated during coolingbeing supplied at least in part by the water added to the system insteadof entirely by the produce. In these approaches known to the inventor,the vacuum cooled produce sprinkled with water has been packed in waxedboxes which absorb very small amounts of water. All of these methods aresignificantly inhibited if product "exposure" is restricted, as when theproduct is packed in a plastic bag; such is the case where modifiedatmosphere packaging is used.

Modified or controlled-atmosphere packaging of fresh produce has alsobeen heretofore utilized and offers advantages to virtually all sectorsof the industry, from grower-shipper to food service and retailconsumers. Benefits include reduced waste due to spoilage, enhancedquality, extended shelf life and greater consumer convenience. Theessential feature of the modified-atmosphere approach to packaging is toseal the product in a package that restricts, to a predetermined degree,the exchange of gases between the product and the surroundings. manystudies have been performed on the desired gas environments for varioustypes of products.

In general, modified-atmosphere packaging retards the four major causesof produce quality loss, namely dehydration, respiration, microbialspoilage and enzyme attack. The quality of cut fruits or vegetables(e.g. florets) deteriorates much more rapidly due to these factors thanif the products remain uncut. Moisture loss from produce is governed byFick's law of diffusion which states that the rate of vapor lossincreases in direct proportion to the vapor pressure difference betweenthe surface of the produce and the surrounding air. Since at a constantrelative humidity, vapor pressure in the air nearly doubles for each 10°C. temperature rise, and vapor pressure at the surface of fresh produceis nearly 100 percent, produce will dehydrate nearly four times fasterat room temperature than at a temperature near freezing, when exposed to"dry" air. A modified-atmosphere packaging with a low moisturepermeability will prevent this loss.

All produce continues to respire after harvest. During normalrespiration, internal carbohydrates are converted into carbon dioxide,water and energy (heat) according to:

(aerobic respiration): C₆ H₁₂ O₆ +60₂ →6CO₂ +6H₂ O+(heat).

This process generally results in a progressive deterioration in productquality. If a harvested item is stored in an oxygen depletedenvironment, anaerobic respiration occurs. This latter type ofrespiration is essentially a fermentation process that results in theproduction of an assortment of organic compounds that lead toundesirable flavors and odors. Anaerobic respiration is described asfollows:

(anaerobic respiration): C₆ H₁₂ O₆ →Alcohols+Acids+CO₂ +H₂ O+(heat).

Aerobic respiration rates can vary greatly among commodities, amongvarieties and even among parts of the same plant. There can be furthervariability due to growing conditions and post-harvest injuries, such asknife cuts, bruises, chill damage, etc. The most significant factorseffecting respiration rate are the stage of maturity of the produce,temperature and storage atmosphere.

The "law of mass action" in chemistry states that the rate of a chemicalreaction is proportional to the concentration of each of the reactants.Thus, aerobic respiration can be slowed by either decreasing the oxygenlevel or increasing the carbon dioxide level of the storage atmosphere.In practice, this relationship appears to hold with the result thatincreasing the CO₂ level is equally as effective as decreasing the O₂level and that the results are additive. Plant sensitivity to CO₂ rangesfrom low tolerance, as with apples, to high tolerance, as withstrawberries.

Enzymes are organic catalysts present in abundance in produce. Afterharvest, these enzymes tend to "spill" from damaged, cut, bruised, etc.cells of produce and can lead to rapid discolorization of light coloredsurfaces, such as of mushrooms and cut apples. There are two basic waysto combat this enzyme activity. The first is through the reduction ofthe oxygen level in a package. Enzymatic browning rate tends to varynearly linearly with oxygen concentration. The second approach is to useenzyme inhibitors. These are components that deactivate the browningenzyme. Sulfite, citric acid and ascorbic acid additives have been usedfor this purpose. In addition, carbon monoxide in concentrations of oneto ten percent is effective as an enzyme inhibiter and as a microbicide.Items known to benefit from small (one to five percent) concentrationsof carbon monoxide include cauliflower, avocados, strawberries,tomatoes, cherries and grapes. Items known to benefit from largerconcentrations (five to ten percent) include lettuce, stone fruit,melons, cantaloupe, mushrooms and citrus products.

Although bacterial diseases can cause significant decay in vegetables,most post-harvest diseases are caused by fungi. Since these organismsrespire in the same manner as the cut plant, their growth in general iscontrolled by the same factors (eg. high CO₂ concentration, etc.). Inaddition, microbial decay is dramatically accelerated under highrelative humidity conditions. There are a variety of chemical treatmentsused to control these pathogens, including carbon monoxide and sulfurdioxide. Related to controlling microbial decay of produce, is thecontrol of insects, in particular with respect to exported productswhich are frequently subjected to quarantine fumigant treatments.

It is also known to inject or charge modified-atmosphere containers withgas of a desired composition for the particular products. This approachhas been used, for example, in connection with bread whereby bread isplaced in plastic wrappers which are injected with gas of the desiredenvironment prior to sealing the bread in the wrappers. In addition,poultry products are packaged in high CO₂ environments and red meatproducts are packaged in high O₂ and CO₂ environments.

Because modified-atmosphere packaging inhibits the action of these majorcauses of product quality loss, it has recently been a focus of muchactivity. In this regard, there is much data which describes the optimalatmosphere for a variety of commodities. For example, the articleentitled "Post-Harvest Technology of Horticultural Crops", by Kader, A.et al, special publication 3311, published by the University ofCalifornia at Davis in 1985, contains a table of optimal storageatmospheres for a wide variety of types of produce. Controlledatmosphere packaging has also been used for bakery, meat and otherperishable food products. In general, it appears that one can deviatesubstantially from an optimal atmosphere and still benefit.Modified-atmosphere packaging is also the subject of numerous patents,such as U.S. Pat. Nos. 4,256,770 to Rainy; 4,515,266 to Myers; and4,910,032 to Antoon, Jr.

Although these technologies exist, when produce is enclosed in amodified-atmosphere package, it becomes difficult to remove heat, suchas heat in the produce and existing at the harvest site or field. Inaddition to this trapped field heat, the produce continues to warm dueto the heat of respiration. As temperature rises, respiration increasesexponentially, resulting in heat build up. This situation can readilylead to a loss of product quality that quickly negates the benefitsintended with the modified-atmosphere package.

In the prior art, due to the fact that controlled-atmosphere packaginginvolves the sealing of products in a package that restricts theexchange of gases between the product and surroundings, conventionaltechniques for field heat removal, such as forced-air cooling andhydrocooling have been applied before the product is sealed in itspackage and palletized. Because the equipment associated with thecooling techniques is usually located at a central location, the use ofmodified-atmosphere packaging systems generally requires that theproduct be shed-packed at a location remote from the picking location,in contradiction to recent trends in agriculture favoring field-packingof many fresh produce items. In addition, if the ready escape of watervapor from the product surface and/or its subsequent flow to a chilledcondensing coil are restricted, the rate of cooling under vacuum may besignificantly reduced, even in the case of otherwise readily-cooleditems, such as lettuce. By their very nature as gas-flow regulatingdevices, typical modified-atmosphere packages would be expected toinhibit the vacuum cooling process, owing to the severely restrictedrates of gas (water vapor) removal from the package.

Thus, the standard modified atmosphere approach for packing berries,such as strawberries, is to pick or harvest the berries into containers;palletize the containers of berries and refrigerate the pallets. Afterthe berries are cooled, the pallets of berries are wrapped in plasticand injected with an enriched C0₂ mixture and shipped. When the palletsreach the distributors or end users, the pallets are broken apart andthe benefit of the modified atmosphere packaging is lost at that point.

For most modified atmosphere packaged produce other than berries, theproduce is harvested and transported to a remote shed for cooling. Thecooled produce is cut, processed and sorted. The cooled and nowprocessed produce is then packaged in a modified atmosphere container.This approach is costly and results in damage to the produce due tomultiple handling steps and due to the delayed placement of the producein a modified atmosphere package.

Therefore, a need exists for a new package and packaging system forovercoming these and other disadvantages of the prior art.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a package forperishable food and horticultural products includes a cooling element. Acontainer of the type providing a controlled flow of gas between theexterior and interior thereof when closed is positioned in thermalcommunication, and preferably in contact with, the cooling element. Thecooling element may comprise a temperature heat sink element inproximity to the exterior of the container for cooling the container.Preferably, the heat sink element is in contact with or adjacent to amajor portion of the exterior surface area of the exterior of thecontainer.

The heat sink element may comprise a chilled or frozen element, such asin the form of a collar. A frozen block of liquid, such as ice, may beused as the heat sink element. Vacuum cooling of the package enhancesthe cooling of the packaged product. Also, frozen or cooled phase changechemicals, such as potassium nitrate or water in a sealed container, maybe used as the heat sink. Also, the heat sink may comprise a coolingelement with a liquid holding portion which contains a liquid and whichis exposed to the environment outside or exteriorly of the containersuch that evaporation of liquid from the liquid holding portion of thecooling element enhances the evaporative cooling of the product packedin the container. In this latter case, vacuum cooling of the coolingelement enhances the evaporation of the liquid from the cooling elementand the cooling of the exterior of the container and of the productspacked therein even though this container is closed to form a modifiedatmosphere environment.

The cooling element may comprise a collar which substantially surroundsthe container. The cooling element and container may be integral. Thecooling element cools the container such that a cold surface is providedwithin the container on which water vapor from products within thecontainer (assuming the products are the type which contain water) maycondense. This accelerates the cooling of the products with or withoutan applied vacuum.

The container may be of a film or films or other material which controlsthe flow of oxygen and carbon dioxide between the interior and exteriorthereof and may also have a water permeable portion so as to permitwater vapor to pass from the interior to the exterior of the container.

In accordance with a specific aspect of the present invention, theliquid holding portion of a liquid evaporative type of cooling elementmay comprise a hydrophilic material, such as a wood pulp sheet. Toincrease the water holding capacity of this material, a superabsorbentmaterial, such as a hydrogel, may be incorporated into the coolingelement. For example, the container may be formed of a hydrophilicmaterial, such as a cellulose based material with cellophane being oneexample such that the container itself holds water used in cooling theproduct. Similarly, the container may be coated with wood pulp or otherhydrophilic material adhered to the container.

To enhance the bulk transfer of gas from the interior of the closedcontainer to the exterior thereof, for example when the container isplaced under vacuum conditions, a bulk gas flow mechanism is providedfor this purpose. In its simplest form, the bulk flow mechanism maycomprise an aperture which is sized to control the flow of gas bydiffusion between the interior and exterior of the container whilepermitting the bulk transfer of gas through the aperture upon subjectingthe container to a vacuum Typical apertures are in the form of a circlewith a diameter of from about twenty-five microns to about six hundredand fifty microns per kilogram of packed product within the container.In another form of a bulk transfer enhancing mechanism, the containerincludes a valve which selectively enhances the bulk transfer rate ofgas from the interior to the exterior of the container, for example,upon the application of a vacuum to the container. Mechanical valves,such as described in U.S. Pat. No. 4,890,637 and used in connection withpackaging coffee, may be used for this purpose. However, a specificpreferred valve is formed by a flexible patch of an oxygen and carbondioxide gas permeable material mounted to the container so as to overlayand close an opening in the container, the container being of asubstantially oxygen and carbon dioxide gas impermeable material. Thepatch may also be of a water vapor permeable material. The patch ismounted to the container at a perimeter surrounding and spaced from theopening in the container. Upon the application of a vacuum to thecontainer, the area of the patch exposed to the interior of thecontainer increases due a bubbling of the patch away from the opening soas to increase the surface area of the patch exposed to the interior ofthe container through the aperture and enhance the bulk transfer rate ofgas through the patch from the interior to the exterior of thecontainer.

As another aspect of the present invention, the package includes areceptacle, which may be of a box-like configuration, for receiving thecooling element and container with these latter components of thepackage being positioned at least partially within the receptacle. Thecooling element may also be integral with the receptacle. In onespecific form, the receptacle comprises a fluted or corrugated core of aliquid resistant material, such as wax impregnated medium, a hydrophilicmaterial at one side of the core so as to form the interior of thereceptacle and a sheet at the opposite side of the core which forms theexterior of the receptacle. The hydrophilic material, which may comprisewood pulp or other suitable material, contains water for cooling theproduce within the container by evaporation. The liquid resistant core,due the corrugations or flutes, provides a path for the flow of airadjacent to the hydrophilic material to aid in the evaporation ofmoisture from the hydrophilic material and thus the cooling of thecontainer. The liquid resistant material also inhibits the transfer ofwater from the hydrophilic material to the cover sheet of thereceptacle. The cover sheet may be printed, for example with brandidentifications or advertising material, such that the entire package issuitable for display in a retail store. In addition, the package may beassembled in the field with the produce being harvested directly intothe container to minimize the handling of the produce between harvestand display.

The receptacles may also be configured for stacking in tiers with theproduct in containers placed in proximity to, preferably in contactwith, the cooling elements and in the receptacles. A vacuum may beapplied to the containers so as to evaporate liquid from products withinthe containers. If the evaporative type cooling elements are used,liquid also evaporates from the cooling elements to cool the productswithin the containers. By making the containers of a flexible material,the containers tend to expand against the respective cooling elementsduring the application of the vacuum to enhance the conductive coolingthrough the container to the cooling elements.

For effective cooling purposes, the liquid absorbent material of theevaporative type cooling element is typically designed for holdingliquid, such as water, in an amount which is at least from aboutforty-five to about sixty-five grams of water for each kilogram ofproduct within the container. Assuming the field temperature of theproducts is approximately 80° F., evaporation of forty-five grams ofwater for each kilogram of the products within the container will causea drop in temperature of about 45° F. in the products, or to 35° F. Theadditional water included in such cooling elements is used to assist inevaporatively cooling the products as they are picked in the field.

The cooling element may include plural passageways open at at least oneend to which gas may pass to enhance the rate of heat transfer, forexample by enhancing the evaporative type evaporation of liquid fromcooling elements. In a specific example, the cooling element may beformed of a corrugated board having a fluted core and a fibrous mat onone surface thereof for purposes of absorbing liquid.

It is accordingly one object of the present invention to provide animproved container for packaging and cooling perishable food andhorticultural products.

Another object of the present invention is to provide a package usablein field applications by which a field-packed modified-atmosphere orother wrapped container may still be effectively cooled, includingcooling under vacuum conditions.

Still another object of the present invention is to provide a packagecapable of enhancing the effectiveness of cooling of a wide variety ofproducts, including strawberries, and in which vacuum cooling may beutilized to enhance the cooling process.

Another object of the present invention is to provide a package whichextends the duration of the peak quality of a product for eating orother use. This allows the picking of produce which is closer to fullmaturity, an expansion of marketing opportunities in that products maybe economically shipped to more distant markets; and an extension of themarket season in that seasonal products may be held longer and still beat high quality when sold.

As another object of the present invention, efficiencies in processingthe products are enhanced and costs are reduced. For example, waste(e.g. lettuce cores, broccoli stalks) can be removed and left in thefield so that the product arrives ready to eat without additionalprocessing being required. This reduces waste disposal costs and laborcosts at the point of sale. In addition, losses due to spoilage of theproducts are reduced. Moreover, transportation costs are reduced as muchof the relatively heavy ice used in the transportation of many types ofproducts, such as broccoli, can be eliminated.

As another object of the present invention, loads of various productsnot otherwise typically shipped together, may be commingled. Forexample, ethylene sensitive products, such as bananas, or odor absorbingproducts, such as strawberries, can be shipped with odor emittingproducts such as onions or ethylene emitting products, such as apples,pears and tomatoes.

As another object of the present invention, the products may be packagedand labeled in the field to minimize the possibility of misbranding ofthe products downstream in the distribution chain.

As an advantage of the present invention, a package is provided whichincreases the room temperature tolerance of the products and enhancesthe duration of peak quality of such products even under such adverseconditions.

As yet another object of the present invention, a package is providedwhich minimizes the possibility of cross-contamination of products, forexample pests found in some products migrating to other products duringshipment.

The present invention relates to the above features, objects andadvantages both individually and collectively. These and other objects,features and advantages of the present invention will become apparentwith reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of one form of package in accordance with thepresent invention illustrating a produce container, a cooling elementand receptacle.

FIG. 2 is a cross-sectional view of a portion of one form of coolingelement in accordance with the present invention also showing a portionof an alternative form of receptacle in accordance with the presentinvention in the event the cooling element and receptacle are combined.

FIG. 3 is cross-sectional view of a portion of an alternative form ofcontainer in accordance with the present invention in which thecontainer and cooling element are combined.

FIG. 4 is cross-sectional illustration of one form of mechanism forincreasing the bulk flow of gas from the interior to the exterior of thecontainer when the container is subjected to a vacuum.

FIG. 5 is an illustration similar to FIG. 4 showing the operation of thebulk gas transfer mechanism when subjected to a vacuum.

FIG. 5(a) is a plan view of the gas transfer mechanism of FIG. 4.

FIG. 6 is a cross-sectional view of a portion of a container whichillustrates an alternative bulk gas transfer mechanism.

FIG. 7 is an exploded view of an alternative form of container inaccordance with the present invention.

FIG. 8 is a plan view of a cutout blank which may be formed into thecooling element of the package of FIG. 7.

FIG. 9 is a plan view of a cutout blank which may formed into thereceptacle of the package of FIG. 7.

FIG. 10 is a schematic illustration of the use of the package in a fieldpacking application.

FIG. 11 is a cross-sectional view illustrating one form of mechanicalfastening mechanism suitable for use in sealing containers of thepresent invention.

FIG. 12 illustrates palletized packages in accordance with the presentinvention and also illustrates heat sealing of the container used insuch packages.

FIGS. 13-15 are graphs illustrating the gas transfer and permeancecharacteristics of selected types of media suitable for use incontainers in accordance with the present invention.

FIG. 16 is a graph illustrating oxygen and carbon dioxide concentrationsachievable in containers of various constructions.

FIG. 17 is an exploded view of a container and another form of coolingelement in accordance with the present invention.

FIG. 18 is a top perspective view of another form of receptacle andcooling element in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The package, packaging system, and method of the present invention isapplicable to the packaging of a wide variety of perishable food andhorticultural products. These products include both respiring andnonrespiring types. Respiring products include, but are not limited to,cut and uncut fruits and vegetables and other horticultural productssuch as cut flowers. Nonrespiring products include, but are not limitedto, bakery products, meats, poultry and fish. Although the invention haswide applicability to the packaging of perishable food and horticulturalproducts in general, the invention offers particular advantages inconjunction with packaging and cooling products, including thoseproducts benefited by a modified atmosphere environment.

For purposes of convenience, and not to be construed as a limitation,the invention will be described in an application involving theharvesting and packaging of strawberries (a respiring product) and inwhich a modified atmosphere environment is utilized.

With reference to FIG. 1, the illustrated package includes a modifiedatmosphere container 10 enclosing strawberries 12 therein, a coolingelement in the form of a cooling collar 14 within which the container 10is positioned when the packaged is assembled, and a box-like receptacle16 for receiving both the cooling collar and container. The illustratedreceptacle 16 is subdivided by a wall 18 into a first compartment 20 anda second compartment 22. Although only one cooling collar 14 andcontainer 10 is shown in FIG. 1, plural such elements are typicallyprovided with one container and collar being positioned in compartment20 and another such container and collar being positioned in compartment22. As explained below, the receptacle 16 is typically of a corrugatedkraft board material assembled to provide reinforced corners and thecentral wall, with upper planar shelves, some being numbered as 26, 28,to facilitate stacking of product containing receptacles on top of oneanother.

The container has a produce containing interior and an exterior and ispreferably of the type which is closable with product to provide acontrolled flow of gas between the interior and exterior of thecontainer when closed. The material used for the container is selectedto provide a desirable gas environment for the particular product beingcontained. Suitable environments and storage conditions are found in theliterature, for example in the previously mentioned article by Kader, A.et al. entitled "Post Harvest Technology of Horticultural Crops." TheKader article mentions that a desirable environment for broccoli is oneto two percent O₂ and five to ten percent CO₂, and that a desiredenvironment for strawberries is ten percent O₂ and fifteen to twentypercent CO₂.

Most gases will dissolve in plastic films. Once dissolved, the gasesdiffuse through the film and eventually evaporate from the oppositesurface. With films, this process has been shown to follow an"Arrehenius" relationship, whereby their permeability increases withtemperature. For most non-gas-barrier films, this temperature changeamounts to approximately doubling the permeability when the temperaturerises from freezing to room temperature. The permeability of a plasticfilm can be increased with the addition of plasticizers. Water vapor isa strong plasticizing agent for hydrophilic polymers, such ascellophane, nylon and ethylene vinyl alcohol; thus, permeabilities ofthese films tend to be highly dependent upon relative humidity.Permeability is somewhat different for each gas depending upon itssolubility and molecular size. Permeability ratios, however, areremarkably constant across a broad spectrum of polymers. As a rule ofthumb, O₂ and nitrogen permeabilities through film are four and eighttimes lower than carbon dioxide, respectively. Each gas diffusesindependent of the others in the mix so that the transfer of a singlegas through a film or membrane is dependent on its partial pressure dropacross the membrane.

Gas permeability of plastic films is measured in accordance with ASTMStandard D1434, commonly referred to as the Dow cell Method. The watervapor transmission rate of plastic films is generally measured inaccordance with ASTM Standard E96. Typical permeance and water vaportransmission data for plastic films can be obtained from the suppliersof these films with one needing only to select a film that provides thedesired environment. In general, the higher the water vapor transmissionrate, the lower the gas permeance of a film. Typical film permeanceproperties of a number of films are set forth in the table below.

                  TABLE I    ______________________________________                                       Permeance.sup.(1)    Film       CO.sub.2                       O.sub.2   N.sub.2                                       WVTR.sup.(2)    ______________________________________    Polyethylene               1,500   270       100   <1    (low density)    Polypropylene               350     100       25    <1    Silicone   350,000 70,000    30,000                                       <1    Cellophane <1      <1        <1    75    Nylon      5       2         <1    10    Polycarbonate               550     150       25    6    Styrene    500     150       30    5    PVC        5,600   550       225   25    ______________________________________     .sup.(1) mL/hr  atm  m.sup.2 (1 mm thickness, room temperature)     .sup.(2) mg/m.sup.2 /day (1 mm thickness, 95% RH, room temperature)

For products which are not sensitive to the presence of water, such asbroccoli, a film container of a material such as polyethylene may beselected. However, for packaging products which are sensitive torelative humidity and the presence of water, for example fruit and sugarcontaining produce such as apples and strawberries, a material with ahigher water vapor transmission rate, such as cellophane is preferred.However, a container entirely of cellophane or of another gas barrierfilm, as is apparent from the above table, would in most cases notprovide the desired controlled atmosphere environment in the containerfor respiring type products as cellophane tends to be a gas-barrier tocarbon dioxide, oxygen and nitrogen.

It should also be noted that for nonrespiring products, barrier typefilms are preferred with the containers being charged with desired mixesof gases during packaging.

A number of options exist for providing a container with a modifiedatmosphere environment and which allows the escape of water vapor. Inone basic approach, the container may be made of more than one material,one of the materials permitting the passage of water vapor and the othermaterial controlling diffusion of gases. This approach, which may becalled a window technique, may be accomplished by, for example, theinclusion of a section or patch of porous or nonporous material in thecontainer, the patch being of the type which controls the desireddiffusion of gas between the interior and exterior of the container.Another approach, as explained below, is to include one or moreapertures in the container which are sized to control the diffusion ofgases through the aperture. As explained below, the use of a patch ofporous material or an apertured container is helpful in vacuum coolingapplications as the apertures and porous material facilitate the bulktransfer of gas from the container when the container is subjected to avacuum.

In connection with the window approach, one container material may berelatively water permeable and a gas barrier, such as cellophane orethylene vinyl alcohol copolymers. Another container material may be anonporous material selected to control the gas transfer by diffusionbetween the interior and exterior of the container so as to establishthe desired controlled atmosphere environment. One approach foraccomplishing this result is to make the container 10 in the form of abag, a portion of which is indicated at 30 in FIG. 4, of a water vaporpermeable gas barrier material with an aperture 32 being provided in thebag. The aperture is covered with a patch 34 having a permeance whichestablishes the desired gas environment within the container. Forexample, the patch 34 may be of silicone such that gas diffuses throughthe patch until the oxygen and carbon dioxide concentrations reach thedesired relative levels within the container. If the product isrespiring, equilibrium levels in the container will differ from air inthat the oxygen concentration is reduced and the carbon dioxideconcentration is increased. Yet, the overall bag material 30 permits theremoval of water and water vapor through this portion of the container.As another option for removing excess water from within the container,desiccants, such as in the form of one or more package inserts, may beincluded within the container.

Referring again to FIG. 4, the patch 34 is typically sealed, as by anadhesive 36 (or mechanically, or heat sealed, or otherwise sealed) tothe container to close the aperture 32. As shown in FIG. 5A, theadhesive 36 is typically placed so as to form a perimeter seal at alocation spaced from the boundary of the aperture 32 for purposesexplained below.

Another window approach involves the use of a porous patch for thewindow. These porous membranes control the bulk diffusion of gas betweenthe interior and exterior of the container so as to control theatmosphere within the container as desired. Examples of suitable porouspatch materials and the measured gas transfer coefficients throughapertures of selected dimensions covered with a number of such porousmaterials are indicated in Table II below.

                  TABLE II    ______________________________________                            Test                            Diameter    Membrane      Condition (cm)      (ML/hr-atm)    ______________________________________    Nuclepore (3 micron)                  Dry       0.69      695                  Wet       0.69      650    Veratec 58.1# Polyester                  Dry       1.0       1,120                  Wet       1.0       880    42# Bleached Liner                  Dry       1.0       155                  Wet       1.0       265    33# Kraft Liner                  Dry       1.0       500                  Wet       1.0       480    Teslin synthetic                  Dry       1.0       655    Paper (PPG) (10 mil)                  Wet       1.0       480    Tyvek #1059B  Dry       5*        7,580    ______________________________________     *Large diameter required due to material nonuniformity.

Yet another way of achieving the desired modified atmosphere environmentwithin the container 10 has been discovered. With reference to FIG. 6,perforating the container 10 with a small aperture or hole 60 has beenfound to work effectively in these applications.

Ordinary molecular diffusion occurs through perforated or porousmembranes whose pore diameters are large relative to the mean free pathof the gas. For atmospheric gases, relatively large pores refers to poresizes larger than about 0.5 microns in diameter. Although ordinarymolecular diffusion increases with absolute temperature to the 1.75power, there is little temperature dependence over the relatively smallrange of interest to modified atmosphere packaging. There is, however, aslight dependence on gas composition, since O₂ and N₂ diffuseapproximately thirty percent more readily than CO₂ and H₂ O vapordiffuses approximately sixty percent more readily than CO₂. However, ithas been found that the gas transfer coefficient increasesproportionately with the circumference of an aperture rather than thearea of the apertures. FIGS. 13, 14 and 15 illustrate these observationsfor three different types of materials. This finding has provided abasis for selecting aperture sizes which result in the desired gasenvironment while still permitting the enhanced bulk transfer of gasunder vacuum conditions. Apertures having an area of that of a circle ofa diameter of from about twenty-five microns to about six hundred andfifty microns per kilogram of packed product have proven to maintain thedesired controlled atmosphere with packages having in the range of up toabout one-half to ten kilograms of packed product having been tested todate.

Another gas transfer mechanism is Knudsen diffusion through porousmembranes whose pore diameters are small relative to the mean free pathof the gas. For atmospheric gases, this means pores smaller than about0.5 microns in diameter. In Knudsen diffusion, gas permeance is relatedto the inverse of the molecular weight of the gas. Thus, theoretically,Knudsen diffusion will result in oxygen and nitrogen permeabilitiestwenty percent and thirty percent higher than carbon dioxide,respectively.

It is also possible to further modify the internal atmosphere of amodified atmosphere container using an assortment of gas scrubbingmaterials. Scrubbing products are commercially available for ethylene,carbon dioxide, oxygen and water vapor. In particular, silica gel andclay are commonly used to scrub water vapor, iron oxide is commonly usedto scrub oxygen, lime is commonly used to scrub carbon dioxide, andpotassium permanganate is commonly used to scrub ethylene from thecontrolled atmosphere environment. In addition, humectants are sometimesused to control the humidity in a controlled atmosphere container.

Designing a modified atmosphere package simply involves throttling theincoming oxygen and outgoing carbon dioxide streams so that respiringproduce becomes starved for oxygen and flooded with carbon dioxide. At asteady state, in general, all of the oxygen being consumed by therespiring produce must pass through the package. This oxygen will passthrough at a rate dependent upon the gas transmission rate of the filmand the partial pressure drop across it. Thus, when respiring produce ispacked in a controlled atmosphere package, the oxygen level willcontinue to drop and the carbon dioxide and water vapor levels willcontinue to rise until the respiration rate is in balance with the gastransfer rate of the film.

As previously mentioned, most plastic films are more permeable to carbondioxide than they are to oxygen. In addition, respiring produce consumesapproximately the same volume of oxygen as the volume of carbon dioxideit emits. Because of these properties, produce in a sealed plastic filmcontainer will reach a stable atmosphere in which the oxygen deficit ishigher than the carbon dioxide buildup. As shown in Table I, permeanceratios (CO₂ :O₂) for "commodity" film materials range from about threeto one (styrene) to ten to one (polyvinyl chloride). With a sealedpolystyrene wrap, it is thus possible to achieve any atmosphere alongthe line AD of FIG. 16. Similarly, with a PVC wrap, one can achieve anyatmosphere along AB. Thus, using the sealed commodity films listed inTable I, it is possible to achieve any atmosphere within the triangleABD of FIG. 16. With other materials, the area within the triangle maybe varied.

Within the ABD range of FIG. 16, the carbon dioxide and oxygen levels donot add to twenty-one percent. This means that a partial vacuum iscreated within the package. As a result, any "pin hole" leak in such apackage will result in nitrogen enrichment to make up the pressuredifference. This in effect provides the basis for an enlargement of thedesign range by using perforated barrier wraps. As previously discussed,if the perforations are large (relative to 0.5 microns), bulk diffusiondominates so that it is possible to achieve any internal atmospherealong AE in FIG. 16. Similarly, if the perforations are small (relativeto 0.5 microns), Knudsen diffusion dominates so that it is possible toachieve any internal atmosphere along line AC.

By combining these mechanisms, (e.g. perforating a gas permeable film)one can obtain any atmosphere within the triangle ABC of FIG. 16.

Thus, a mechanism is described for readily selecting materials forobtaining a desired controlled atmosphere environment for a wide varietyof products.

By circumventing the inherent restrictions placed on the outward watervapor flow by modified-atmosphere packages, effective cooling ofproducts sealed in such packages is permitted. This cooling isaccomplished by locating a heat sink or cooling element in proximity tothe outside wall of the container 10. The heat sink is in close enoughproximity to the outside wall so as to facilitate heat transfer betweenthe exterior of the container and the heat sink. Most preferably theheat sink is in direct contact with such outside wall of the container.It is also preferred that the heat sink contact is proximate to a majorportion of the surface area of the exterior of the container. A majorportion means for purposes of this description at least from aboutthirty to about fifty percent of the exterior container wall surfacearea.

To facilitate use of the invention in the field, it is preferred thatthe package, including the cooling element, be of a size and weightwhich makes them easily manually portable. Consequently, harvesters cancarry these packages with them as they move about a field and harvestproducts. Typically, the receptacles, containers, cooling elements andpacked produce in a package of the present invention weigh less thanfifty pounds to facilitate manual carrying of the package.

The cooling element may take many forms to produce the desired heat sinkat the exterior of the container. As shown in FIG. 17, the coolingelement may comprise a collar 14b, such as of a liquid impermeableplastic film (e.g. polyethylene) which seals a liquid therein, such aswater or a phase change cooling chemical, with potassium nitrate beingone example. The illustrated collar 14b has plural liquid containingcompartments 15a, 15b, 15c and 15d which are joined together by hingeforming portions, such as indicated at 17. The components may be formed,for example, by sealing the outer pouch forming cover material togetherat the hinge locations, to separate the collar 14b into the individualcooling material containing compartments.

The collar 14b is typically frozen and placed in a receptacle 16 withthe container 10 opened and positioned within the collar in the field.Produce is harvested into the container. When the container is full, thecontainer is closed, as explained below, for example in the field. Thecollar 14b acts as a heat sink to cool the exterior wall of thecontainer and the packed product, which may then subsequently be vacuumcooled to substantially accelerate the cooling, such as explained byexample in conjunction with Table III, below. The collar 14b may beremoved prior to shipment of the cooled produce and refrozen forsubsequent reuse in the field harvesting operation. Other forms of heatsinks or cooling elements may also be used, provided the heat sinkoffers a sufficient thermal mass to accomplish the desired cooling.Preferably the thermal mass is such that it is capable of dropping thetemperature of packed produce in the field under normal fieldtemperatures and container filling times from about 80° F. to about 35°F. In general, it is preferred that the thermal mass be capable of oneBTU (British Thermal Unit) per pound of packed product in the containerper degree of cooling desired. Thus, for a temperature drop of from 80°F. to about 35° F., the thermal capacity of the collar preferably is atleast about 45 BTU per pound of packed product by providing excessthermal capacity (e.g. another 5 BTU per pound of packed product), thecooling collar also compensates somewhat for the time the container isexposed to field temperatures as the container is being filled.

The cooling element may also be of the type which permits theevaporation of liquid therefrom to provide the heat sink in this manner.Evaporation of a cooling liquid in proximity to the exterior wall of thecontainer 10 cools the contents of the container by evaporation andtransfer of heat from the products in the container through thecontainer wall. Water vapor within the package, for example, frommoisture containing products, tends to condense on the chilled innersurface of the package wall (which is also true when other types ofcooling elements are used), reducing the water vapor pressure inside thepackage and promoting further evaporation of water from the moistproduct. This evaporation from both the cooling element and packedproduct is enhanced under vacuum conditions and results in a rapidcooling of the product. Thus, cooling, and in particular a vacuumcooling approach can be applied to the product within amodified-atmosphere package. Cooling is accomplished by a seriesevaporation-condensation-evaporation process that is facilitated by themoisture source in proximity to or contact with the exterior containerwall.

Although FIG. 1 illustrates one form of a separate cooling element whichis capable of holding a volatile liquid, such as water, ethanol or thelike, against the container wall, other approaches may be used. Forexample, by making the container of a hydrophilic material, such as of acellulose based material (e.g. nylon, cellulose acetate, cellophane orother dissolved cellulose based films) or other absorbent material, thecontainer 10 itself may function as a cooling element with liquidevaporating from the container to facilitate the cooling of itscontents. Polysaccharide films, hydrogels (such as the so-calledsuperabsorbent particles common in the disposable diaper art) adhered tofilm, fibrous materials such as wood pulp adhered to the film, waterpouches or pockets on the container, are yet other examples ofmechanisms for incorporating liquid into the container for purposes ofevaporative cooling. For example, FIG. 3 illustrates a film 30 withadhered wood pulp particles 40, the wood pulp particles holding waterfor use in evaporative cooling of the contents of the container.

The required capacity of the moisture source, whether it be a substrateon the container 10 or moisture holding substrate in a separate coolingelement such as collar 14, depends upon the mass of the product withinthe package. With water being the cooling liquid, a rule of thumbindicates that one percent of the product mass is lost to evaporationfor every 10° F. of vacuum cooling. To minimize evaporation of moisturefrom the product itself during cooling, the moisture source is typicallydesigned to provide at least this minimum mass. In a typical fieldpacking operation, one can assume an average air temperature of about80° F. Therefore, to drop the temperature of products from 80° F. to 35°F. would require about forty-five grams of water for each kilogram ofproduct in the container. However, in accordance with the method of thepresent invention, and to gain benefits of cooling during harvesting ofthe produce, water is typically added to the cooling element or packagein advance of harvesting the produce such that the produce is harvestedinto a container already provided with this added moisture. Becauseevaporation can take place, and is encouraged for cooling purposes,during actual picking of the strawberries or other products, excesswater is typically included so that enough water remains in thecontainer for purposes of subsequent evaporative cooling, such as undervacuum conditions. Therefore, a preferable cooling container is designedto hold an excess amount of water, such as about sixty-five grams ofwater for each kilogram of product in the container. Also, in general,the greater the proportion of the container in contact with the moisturesource, the more effective the cooling. In addition, relatively thinmoisture containing substrates offer a low resistance to the transfer ofheat from the condensing surface at the interior of the container to theevaporating moisture in the substrate and thereby increase coolingeffectiveness.

To accommodate this relatively large quantity of moisture, the moistureis most conveniently placed in a substrate with the substrate beingpositioned in contact with the container wall. Also, by utilizing acontainer 10 of a flexible material, the container expands against thesubstrate during the application of a vacuum. This advantage is alsoobtained by using a flexible container with the other forms of coolingelements. This is due to the delay in evacuating the air from thecontainer and the fact that the container tends to inflate against thecooling element, thereby enhancing the contact between these componentsand enhancing the resulting heat transfer.

Any moisture absorbing material may be utilized, such as blotter pads,absorbent fluff pulp, superabsorbent polymers, paper, molded fiber andcombinations thereof. The location of the moisture containing substratewith respect to the container 10 may be varied, such as underneath,along side, or on top of the container.

In the design of a cooling element such as collar 14 shown in FIG. 1,the substrate material is indicated at 50 and positioned at the interiorof the collar 14. In FIG. 2, the water containing substrate 50 comprisesa sheet which is positioned at a surface of the collar 14 and which isincorporated into the collar. Again, the sheet may be of any suitableliquid containing material, such as wood pulp. Also as shown in FIG. 2,the collar 14 may include a conventional corrugated core, indicated at52, such as of corrugated Kraft paper. The corrugations definepassageways or flutes, some being indicated at 54 in FIG. 2, whichpermit the passage of air or otherwise expose the back side of the sheet50. Consequently, evaporation of liquid from the back side of the sheetis enhanced. This can be important, especially if the container ispressing against the exposed surface 56 of the sheet so as to limitevaporation at the area of contact between the container and sheet. Tolimit the possible transmission of liquid to an exterior sheet 58 of thecollar 14, the core 52 may be formed of a water resistent or waterimpermeable material. Wax impregnated medium, such as a waxed paper, isone specific example of a medium which may be utilized for this purpose.Although migration of liquid through the liner and the core 52 to thesheet 58 is typically limited in any event, the use of a water resistentcore 52 minimizes the potential wetting of the sheet 58.

The receptacle 16 may be a separate element as indicated at FIG. 1, ormay be combined with the cooling element 14. One convenient approach forcombining these elements is to utilize the structure of FIG. 2 for thereceptacle, in which case the interior surface of the receptaclecomprises the water holding or carrying material, such as the sheet 50.Also, with a water resistent core 52, the sheet 58 remains substantiallydry. Therefore, the sheet 52 may be preprinted with brand identificationor other advertising material so that the receptacle 16 is usable as thedisplay container for the produce, such as in a retail establishment. Ofcourse, a separate receptacle 16 may also be used for this purpose. Withthe optional construction utilizing a water resistent core 52, thereceptacle 16 remains strong enough for stacking and carrying theproducts as well as for protecting the products during shipment eventhough the sheet 50 is wet.

In applications wherein the package is to be vacuum cooled, cooling isgreatly assisted if a path is provided for removal of air from theinside of the package during the evacuation period. Otherwise thepressure of air within the package inhibits the condensation of watervapor from the product onto the cold package wall. One way of providingthe pathway is to utilize the small window or patch of porous filtrationmaterial which allows the bulk transfer of air from within the containerduring the application of the vacuum while still permitting diffusion tocontrol the gas balance within the container during storage. However, toincrease the gas transfer rate during evaporative cooling, mechanicalvalves, such as the valve described in U.S. Pat. No. 4,890,637 or thelike, may be included in the wall of the container 10. Althoughsuitable, mechanical valves tend to add to the expense of the packagingsystem.

As another approach for increasing the bulk transfer of air from theinterior of a container under vacuum conditions, reference should bemade to FIGS. 4, 5 and 5a. As previously explained, the patch 34 istypically secured, as by adhesive, to the container wall 30 about aperimeter 36 which is spaced from the boundary of the aperture 32. Undervacuum conditions, the patch 34 tends to form a bubble, as shown in FIG.5, whereas in the absence of the vacuum, the patch tends to lay flatagainst the container wall as shown in FIG. 4. In comparing FIGS. 4 and5, it is apparent that the area of the underside of the patch 34 exposedto the aperture 32 is increased under vacuum conditions as opposed tothe case when a vacuum is not being applied. Due to the increase inexposed area of the patch 34, the gas transfer rate through the patch 34is increased under vacuum conditions. Consequently, a more rapid escapeof air from within the container is permitted when a vacuum is appliedand, as a result, more effective cooling of the product containedtherein takes place.

Also, the use of an aperture in the container (See FIG. 6) enhances thebulk gas flow under vacuum conditions.

In connection with bulk flow of gases, gases are transferred from thehigh pressure side of the package to the low pressure side independentlyof the partial pressure differences of each gas component. For example,if air is bulk transferred from the outside of a package to the inside,enrichment is in the constant ratio of seventy-nine parts nitrogen totwenty-one parts oxygen (the composition of air), regardless of what theinternal partial concentrations of these gases are.

As previously mentioned, the package of the present invention can beutilized in conjunction with various means of achieving evaporativecooling. For example, water vapor may simply be allowed to evaporatefrom an evaporative type cooling collar. In addition, affirmativeevaporative cooling may be accomplished by moving air across such acooling collar. Pressure cooling may also be utilized, involving use ofdry air at a higher temperature. In addition, and offering particularadvantages, vacuum cooling may be employed to cause the flashing ofwater vapor from the produce and from an evaporative type cooling collarwhen air is removed as a vacuum is applied.

It is also possible to charge the package with a desired gasenvironment. For example, the vacuum may be relieved by charging thevacuum chamber with a desired gas atmosphere having a gas balance whichdiffers from air. For a nonrespiring product in a gas barrier film, themodified atmosphere within the container remains at the charged gascomposition for a substantial period of time. For example, theatmosphere may be enriched in carbon dioxide. This charging gas willpass into the container and effectively precharge the container with gasof the desired environment. The charging gases may include a fumigantfor destroying fungi, bacteria, insects and other pests that mightotherwise damage the packaged product. A number of known fumigants canbe used, such as methyl bromide gas for mite control to satisfy exportrequirements, such as the case for strawberries being shipped to anumber of foreign countries. In addition, gases such as carbon monoxidemay be used to inhibit enzymes responsible for browning of lettuce,mushrooms and other products. Again, any number of suitable fumigantsmay be utilized, with other examples including sulfur dioxide andsulfite based materials. Other chemicals for these purposes may be addedin liquid or solid form.

FIG. 7 illustrates another form of package in accordance with thepresent invention with corresponding elements being assigned the samenumbers as in FIG. 1, but with the added subscript "a". In this case, asomewhat smaller container 10a, in comparison to the container 10 ofFIG. 1, is shown with strawberries 12a therein. The cooling collar 14ain this case is formed into a box-like configuration with a waterabsorbing substrate 50a at one surface of this form of cooling element.In the FIG. 7 package, the receptacle 16 is comprised of a firstreceptacle 16a for receiving the container 10a and cooling collar 14atherein and a larger receptacle 16b for receiving plural, in this casefour, of the containers 16a and contents.

FIG. 8 illustrates a corrugated board blank used in forming the coolingcollar 14a of FIG. 7. When folded along perforations 60, 62, 64, 66, 68and 70, the cooling collar 14a takes the form of a box which may includethe water holding substrate 50 on all of its interior surfaces. Duringuse, the collars 14a, as well as collars of the form 14 shown in FIG. 1,and 14c in FIG. 18, are typically inverted (substrate 50, 50a, 50c sidedown) and floated in a pool of liquid, such as water, so that thesecollars become at least partially saturated. To expedite this wettingprocedure, the blanks used to form the collars 14, 14a and 14c may becarried by a conveyer across the surface of a pool of water with thesubstrate 50 in contact with the water so as to wet the substratewithout wetting the remaining surfaces of the collar. However, theentire collar may be wetted if desired.

FIG. 9 illustrates a corrugated board blank for one of the receptacles16a which, if folded along perforations 80-94 forms another box-likestructure for receiving the cooling collar and container.

With reference to FIG. 10, a typical method in accordance with thepresent invention will be described. In this case, at a location 100 acooling liquid, such as water, is added to the cooling collar. This maybe accomplished by at least partially saturating the substrate 50 of thecooling collars 14, 14a, or 14c (FIGS. 1, 7, 18). Liquid is typicallyadded to the cooling collars in the field, that is at the location wherethe products are to be harvested. Following the addition of the coolingliquid, (or in the case of the collar 14b folling the chilling orfreezing of the collar) the packages are typically assembled. That is,open containers 10, 10a are placed in respective cooling collars 14,14a, 14b, 14c and in the receptacles 16 or 16a and 16b. The assembledcontainers, one being indicated at 102 in FIG. 10, are then taken by theproduce harvesters and filled with produce, such as strawberries from arow 104. The strawberries are sorted by the picker and placed directlyinto the open containers 10, 10a. Evaporation of liquid from the coolingcollars 14, 14a, 14c (and heat transfer to the chilled collar 14b ifthis type of collar is used) helps to cool the berries as they are beingharvested.

In a typical commercial strawberry field, plastic or other groundcovering 106 is placed on the ground between the plants so that theberries are clean. Thus, the berries being placed in the containers 10,10a are clean and attractive for marketing purposes. The picker, whencontainers 10 and 10a are full, typically takes the filled package to asealing location, indicated at 108, at which time the controlledatmosphere packages 10, 10a are closed.

The containers may be provided with mechanical fastening mechanisms foruse in sealing the containers. One such mechanism is shown in FIG. 11and is indicated by number 110 as comprising a common "zip-lock" typemechanism having an elongated bead 112 which fits within and mates withan elongated groove 114 formed in the container 10. This mechanism maybe provided in a strip of material secured to the container. Althoughmechanical seals may provide the sole sealing for the containers 10,10a, films of this type are typically of a heat sealable material.Consequently, as shown in FIG. 12, a filled package 102 may simply beplaced on a table 116 with the open end of the container 10, 10a beingexposed for positioning between heating elements 120, 122 of anelectrically powered heater 124. With the end 118 of the bag claspedbetween the bars 120 and 122, the bag is closed by heat sealing. ofcourse, ultrasonic and other sealing approaches may also be used. Forexample, commercially available cable ties, such as Part No. 95476 tiesfrom Consolidated Plastics Company of Twinsburg, Ohio have provensuitable. In addition, the mechanical fastening mechanism 110, althoughhelpful in preliminarily closing the bags so that ends 118 may beoriented easily for heat sealing, is not necessary. After sealing, thenow sealed end of the bag 118 is typically tucked into the receptacles.

As shown in FIG. 12, the receptacles may be printed with brandidentifying indicia or advertising material, as indicated at 130, sothat the produce can be displayed at its end destination, such as atretail stores, in these receptacles. As also shown in FIGS. 10, 12,following sealing, the packaged products may be palletized, that is,stacked in tiers on a pallet 138 as shown in FIG. 12. This approachminimizes the number of times that the produce is handled followingharvest. That is, the only direct handling of the produce occurs at thetime it is picked and initially placed in the container and then againat the restaurant or other end location when the produce is actuallyused. Also, the modified atmosphere container typically remains in tactuntil the individual containers of produce are used. Although theproduce has been placed in modified atmosphere containers, evaporationof liquid from the form of cooling collars 14, 14a continues to cool theproduce. In a like manner, heat transfer to the FIG. 17 form of coolingcollar 14b also continues to cool the produce.

Following the optional palletizing step, the packaged product is movedto a vacuum cooler of a conventional type. The vacuum cooler may belocated at the field, that is in proximity to the location where theproduct is harvested, or at a remote site. The packaged product issubjected to vacuum cooling to further cool the product untiltransported, as indicated by vehicle 142 in FIG. 10, during distributionof the product.

Finally, to provide a further explanation the present invention, aspecific example is described below. In connection with this example, afour-unit retail flat of the type shown in FIG. 7 was used. Eachcontainer 10a of this flat was packed with approximately one thousandgrams of strawberries. The film utilized in the container 10a wasethylene vinyl alcohol (EVOH) having a twelve micron thickness and beingapproximately of a twelve inch by five and one-half inch by six inchsize. The patch 34 (FIG. 4) comprised forty-two pound bleached linerpaper in the form of a one and one-fourth inch by one and one-fourthinch label with a one-quarter inch diameter adhesive-free circular areaapplied positioned over a one-sixteenth inch diameter perforation in thefilm (the perforation corresponding to aperture 32 in FIG. 5). Over theaperture, the gas transfer coefficient (diffusion) was measured as 80mL/hr/atm while the Gurly (bulk flow) was measured at 560 sec/100 mL. Inaddition, the cooling collar 14a (FIG. 7) was partially saturated withapproximately 100 grams of water with the assembly being placed in oneof the containers 16a. Testing revealed the steady-state internalatmosphere of this container was approximately seven percent CO₂ andsixteen percent O₂ at 40° F.

When a package of this type including a cooling collar is stored in awell-ventilated area, the temperature of the cooling collar approachesthe wet bulb temperature of the surrounding air. For example, inWatsonville, Calif., where the average high temperature in June is 70°F. and the average relative humidity is fifty percent, the wet bulbtemperature is approximately 60° F. It has been found that after twohours under these conditions, strawberry packages with a cooling collaras described above are on the average 3.5° F. cooler than those withoutcollars.

When subjected to a vacuum, to minimize bursting problems of thecontainer 10a, the porous membrane typically has a Gurly flow of greaterthan 0.2 mL/sec (100 mL/560 sec). This Gurly flow is also achieved byplacing an oversized porous label, for example one-quarter inch indiameter, over a one-sixteenth inch diameter perforation in the film. Aspreviously explained, under vacuum conditions this label bubbles out toexpose the entire one-quarter inch diameter porous material, but thenreturns to a flat position under ambient conditions. Also, as previouslyexplained, a small aperture may be used for this purpose.

In a conventional vacuum cooling process (e.g. no cooling collar orother cooling element), all of the heat removed from a product iscontained in the water vapor and is removed from the product or from anywater sprayed onto the product. With a modified atmosphere package, theremoval rate of heat from the product would therefore be limited by therate at which water vapor would pass through the porous membrane, whichin turn is related to its Gurly number, or to the rate water vapor isotherwise collected within the container. If a cooling collar is used, acondensing surface is created on an interior surface of the modifiedatmosphere container. This allows the water vapor inside the containerto give up its heat (while condensing) to the cooling collar so as toenable a much more rapid heat transfer. In addition, the cooling collarremoves heat by conduction at points of contact with the container. Ithas been observed that after either a fifteen minute or thirty minutevacuum cycle, the temperature drop of a package which combines amodified atmosphere container with a cooling collar is three to fourtimes greater than the case without a cooling collar. In addition, ithas been observed that this method of cooling (utilizing a coolingelement in combination with a modified atmosphere package) appears to begentler on strawberries than a conventional vacuum cooling process.Although the reason is not entirely clear, it is quite possible thatevacuation shock and cell rupture of the berries is reduced and thatfreezing is minimized since no berry can be colder than the coolingcollar.

Also, after about fifteen minutes in a vacuum tube, (an open, e.g.conventional modified atmosphere package) would be approximately 60°cooler if a cooling collar is used than if one is not used. Thus, acooling collar may be used to speed up the cycle time of vacuum cooling.In addition, with such a cooling collar, cooling has been observed tocontinue for several hours after removal from the vacuum tube as heatcontinues to transfer to the collar. Early observations suggest that theequilibrium (two-hour) temperature drop using a liquid evaporative typecooling collar in combination with a fifteen minute vacuum cycle iscomparable to that from the use of a thirty minute vacuum cycle withouta cooling collar.

Mature (full color) strawberries packaged in this manner have maintainedtheir peak quality for eating up to three weeks from packaging.Presently, the maximum strawberry life is about seven to ten days evenif the berries are less mature when picked (green). Although thisexample has been described in connection with strawberries, theinvention is not limited to this particular type of produce. As anotherspecific example, broccoli packaged in this manner has maintained itsquality and freshness for the duration of a twenty-one day test period,with the maximum duration not yet having been determined.

To provide further evidence of the effectiveness of the presentinvention, room temperature strawberries were placed in a modifiedatmosphere container and the container was closed. The container wasthen subjected to cooling for thirty minutes under conditions indicatedby Table III below and with the results being set forth in this table.

                  TABLE III    ______________________________________    STRAWBERRY COOLING IN A    CLOSED MODIFIED ATMOSPHERE CONTAINER    (EVOH Film with an Aperture)                    Vacuum     Average Packed Straw-    Cooling Element Applied    berry Temperature    Type        Area*   During Cooling                                   Initial                                        Final                                             Change    ______________________________________    Frozen Sealed Cooling                120     No         66.6 52.0 14.6    Collar (14b type)**    Frozen Sealed Cooling                120     Yes        65.0 36.8 28.2    Collar (14b type)**    Wet Fiber Cooling                50      Yes        67.8 37.2 30.6    Collar (14a type)    None                Yes        66.9 59.6 7.3    ______________________________________     *In square inches per pound of packaged fruit     **Commercially available Blue Ice ™ packaged sealed cooling elements     frozen at a temperature of 15° F.

From the above table it is apparent that vacuum cooling of roomtemperature strawberries is simply ineffective without a cooling collar.Also, the use of a sealed cooling element without vacuum cooling of thestrawberries offered an improvement over the cooling element-less vacuumcooling approach. Moreover, the combination of vacuum cooling with acooling element (of either the sealed or evaporative cooling type) wasextremely effective in cooling the strawberries. Also, the evaporativecooling type of cooling element required far less surface area than thesealed type cooling element to accomplish the substantially same result.

Having illustrated and described the principles of our invention withreference to several preferred embodiments, it should be apparent tothose of ordinary skill in the art that the invention may be modified inarrangement and detail without departing from such principles. We claimas our invention all such modifications which fall within the scope ofthe following claims.

We claim:
 1. A package manually transportable by a single individual forperishable food and horticultural products comprising:a frozen coolingelement; a container having an interior and exterior, the containerbeing closable with products therein, the container restricting theexchange of gases between the products within the container and theexterior of the container to provide a modified atmosphere environmenttherein of an increased carbon dioxide concentration and a decreasedoxygen concentration relative to carbon dioxide concentration and oxygenconcentration in air when the container is closed and the containerproviding a controlled flow of gas between the interior and exterior ofthe container when the container is closed, the exterior of thecontainer being positioned in proximity to at least a portion of thefrozen cooling element; the cooling element substantially surroundingthe container and which is in contact with at least thirty to fiftypercent of the exterior surface of the container; a receptacle, thecooling element and container being positioned at least partially withinthe receptacle; and the cooling element, container, receptacle andproducts in the container being manually transportable by a singleindividual.
 2. A package according to claim 1 in which the coolingelement comprises a sealed element sealed separately from the containerand containing a solid to liquid phase change material.
 3. A packageaccording to claim 1 in which the cooling element comprises a coolingcollar which is capable of absorbing one BTU per pound of product in thecontainer per degree of cooling of the product.
 4. A package accordingto claim 3 in which the cooling collar is capable of absorbing at least45 BTU's per pound of product in the container.
 5. A system of packagingfresh horticultural products for manual transportation, the systemcomprising:a receptacle; a controlled atmosphere product receivingcontainer having an interior and an exterior and being positioned in thereceptacle, the container restricting the exchange of gases betweenproducts within the container and the exterior of the container toprovide a modified atmosphere environment therein of an increased carbondioxide concentration and a decreased oxygen concentration relative tocarbon dioxide concentration and oxygen concentration in air when thecontainer is closed, the container providing a controlled rate of gasflow between the interior and exterior thereof when the container isclosed; a cooling element positioned within the receptacle andexteriorly of the container for cooling the container to thereby coolproducts in the container; and means for applying a vacuum to thecontainer and cooling element so as to evaporate liquid fromhorticultural products within the container to cool the horticulturalproducts within the container.
 6. A system according to claim 5 in whichthe cooling element comprises a sealed element containing a solid toliquid phase change material.
 7. A system according to claim 5 in whichthe cooling element comprises a heat sink in proximity to a majorportion of at least thirty to fifty percent of the exterior surface areaof the container.
 8. A system according to claim 5 in which the coolingelement comprises a heat sink in contact with a major portion of atleast thirty to fifty percent of the exterior surface area of thecontainer.
 9. A package according to claim 1 in which the coolingelement has at least fifty square inches of surface area per pound ofpackaged product.
 10. A system according to claim 5 in which the coolingelement has at least fifty square inches of surface area per pound ofpackaged product.
 11. A system of packaging fresh horticultural productswhich are harvested in a field where they are growing, the systemcomprising:a receptacle; a cooling element positioned within thereceptacle for cooling product placed in the receptacle, wherein thereceptacle, cooling element and product packed therein are sized formanual transportation by a single individual in the field; and a meansfor applying a vacuum to the receptacle, cooling element and productplaced in the receptacle to accelerate the cooling of such product. 12.A system according to claim 11 including plural receptacles, eachreceptacle containing a respective cooling element, andwherein the meansfor applying a vacuum comprises means for simultaneously applying avacuum to the plural receptacles, and to the cooling elements andproduct contained in such receptacles.
 13. A system according to claim11 in which the cooling element comprises a cooling collar.